V4L/DVB (13015): kmalloc failure ignored in m920x_firmware_download()

Prevent NULL dereference if kmalloc() fails.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>